The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Internal combustion engines combust an air and fuel mixture within cylinders to produce drive torque. A byproduct of combustion is exhaust gas. The exhaust gas may include various components, such as nitrogen oxides (NOx), carbon monoxide (CO), and hydrocarbons (HC). An exhaust treatment system includes a catalyst that converts the NOx, CO, and HC to carbon dioxide and water.
Conversion efficiency of the catalyst refers to the catalyst's ability to react with or convert one or more components of the exhaust gas. The conversion efficiency of the catalyst is related to the temperature of the catalyst. The catalyst may operate at a reduced conversion efficiency when the catalyst temperature is less than a threshold temperature, which is referred to as a light-off temperature. The catalyst's conversion efficiency may be increased by regulating the catalyst temperature at or above the light-off temperature.
The catalyst temperature may be increased using various methods. For example only, heat from the exhaust gas expelled from the engine may increase the catalyst temperature. The exhaust gas transfers heat to the catalyst via convection, thereby increasing the catalyst temperature. Fueling to the engine may also be adjusted to increase the catalyst temperature. For example only, unburned HC resulting from combustion may enter the catalyst where the HC oxidizes and increases the catalyst temperature.
Hybrid vehicles generally include the internal combustion engine and an electric motor. The electric motor may be used for propulsion during city driving where vehicle kinetic energy can be recovered by regenerative braking and stored in an energy storage device. The recovered energy can later be used to drive the electric motor.
The internal combustion engine may be more suitable during highway driving, during which braking and opportunities for energy recovery are infrequent, and the engine operates at its greatest efficiency. Accordingly, the internal combustion engine may be used for vehicle propulsion during highway driving. In mixed city and highway driving conditions, the electric motor may be used together with the combustion engine.